Stretchy pressure-sensitive material could serve as robot skin

Robots, prosthetic limbs and touchscreen displays could all end up utilizing technology recently developed at California's Stanford University. A team led by Zhenan Bao, an associate professor of chemical engineering, has created a very stretchy skin-like pressure-sensitive material that can detect everything from a finger-pinch to over twice the pressure that would be exerted by an elephant standing on one foot. The sensitivity of the material is attained through two layers of carbon nanotubes, that act like a series of tiny springs.

The sensor material is made by first spraying nanotubes in a liquid suspension onto a thin layer of transparent silicone. Although the nanotubes are initially deposited in random clumps, some of them align with one another when the silicone is stretched for the first time, in one direction. Even after the material is allowed to rebound back to its original size, the clumps remain aligned, and will do so indefinitely.

When the silicone is then first stretched in a direction perpendicular to the previous one, some more of the clumps align with one another, facing in that direction. The result is a sheet of silicone coated with "nano-springs" that can be stretched in any direction, that will retain their orientation through repeated stretchings.

Two of these sheets are joined together face-to-face, with the nanotube clusters on the inside. Between them, however, is a third layer of silicone, which is more malleable. This layer stores an electrical charge, like a battery.

When the three-layered material is subjected to external pressure, the middle layer compresses, altering its electrical charge. The conductive nanotube coatings on either side of it, acting like the positive and negative terminals on a battery, detect this change. In this way, the material is able to register not only the fact that it is being pressed, but it can quantify the amount of pressure that it is being subjected to.

Complicating things is the fact that the middle layer will become thinner when the material is being compressed and when it's being stretched. The pattern of the pressure, however, should make it possible to deduce what's going on - compression tends to take the form of a spot of pressure, whereas stretching results in a line of pressure between two points.

Another transparent, stretchable pressure-sensitive material was recently demonstrated by scientists from Germany's Fraunhofer Institute of Silicate Research. It also incorporates electrodes that detect changes in electrical capacitance.